The known veneer lathes may be divided into two types: a conventional veneer lathe in which all the driving power required for cutting a log is supplied to the axial portion of a log only through spindles, and a peripheral driving type veneer lathe in which a peripheral driving mechanism is provided for supplying at least a part of the driving power required for cutting the log to the outer circumferential surface of a log through a peripheral driving member of an appropriate shape which is engaged with the outer circumferential surface of the log immediately before the cutting operation (just in front of the tip of a tool for the cutting) as disclosed, for example, in Japanese Patent Publication No. 16729/1981, and Japanese Patent Publication No. 27161/1988.
Irrespective of the types, however, it is desirable to have the so-called double spindle comprising two spindles, large and small, being interchangeably applied as required in order to securely hold logs of various sizes for cutting.
Nevertheless, the conventional double spindle is so structured that the two spindles, large and small, are rotated together with the small spindle being inserted in the inside of the large spindle in such a manner that it is relatively transferred freely in the axial direction but not relatively rotated. Therefore, although their structure is simple, the total inertial resistance is great, making it unsuitable to use them at a high-speed rotation simply because such a high-speed rotation with a great inertial resistance cannot be controlled with ease. As a result, they have a disadvantage in that the operation becomes less economical and productive.
In other words, the conventional veneer lathe, for example, needs an arrangement of driving devices of large capacity in its driving system because it is required to rotate said double spindle always against the total inertial resistance thereof. Naturally, therefore, the power consumption becomes uneconomically great. In addition, when the small spindle is in use, the large spindle and its driving system having an extremely great inertial resistance must be rotated simultaneously. Consequently, the incremental ratio of revolution has to be kept low or the maximum revolution should be limited to a lower level leading to a reduction in the machining rate of veneer, and the productivity becomes low after all.
The same situation exists in the veneer lathe of said peripheral driving type. For example, when the double spindle is positively rotated, the arrangement of driving devices and the power consumption are not economical. The problem of a lower productivity still exists when the small spindle is used. Particularly in view of rationalization of processing veneer, it is desirable to get a constant speed of single board machining by adopting a structure in which the driving speed of a log in the peripheral driving device is set at a constant speed and further the double spindle is driven rotatable under the revolution corresponding to the log-driving speed. But because difficulty in exerting following control of the revolution in the high speed ratio region it is necessary to select a less than optimum speed.
More precisely, if the machining speed of veneer is assumed to be set, for example at 110 m/min., the revolution of a spindle required to hold a log of 28 cm diameter is approximately 125 rpm, and likewise, if the diameter of a log is 25 cm, approximately 140 rpm, if the diameter is 22 cm, approximately 160 rpm, if the diameter is 19 cm, approximately 185 rpm, and if the diameter is 16 cm, it is approximately 220 rpm. Therefore, when the large spindle (usually 18 cm to 15 cm in diameter) is used, its absolute value and relative incremental ratio are not so large. But at 13 cm diameter, the required revolution becomes approximately 270 rpm, at 10 cm diameter, approximately 350 rpm, and at 7 cm diameter, approximately 500 rpm, so that when the small spindle (usually 8 cm to 5 cm in diameter) is used, the absolute value and relative incremental ratio become greater rapidly.
And if the small spindle of a conventional double spindle is rotated with the large spindle in a manner such as to follow said required revolution, it is needed to control driving devices of larger capacity precisely and, the more the revolution region becomes, the more conformingly, over a wider range of speed changes. To meet these requirements using generally available devices and techniques is extremely difficult. If, for example, revolution of the small spindle is increased beyond a required revolution, the driving force supplied to the axial portion tends to become too great, resulting in more breakage of the axial portion of a log. On the other hand, if the revolution is decreased below the required one, the driving force supplied to the outer circumferential surface tends to become too great, resulting in more damage to the surface of the log (veneer). In practical operation, such undesirable results as these should be prevented anyway.
Therefore, in order to meet the above-mentioned requirements using generally available devices and techniques, the absolute value of the required revolution is limited within a range where a follow-up rotational driving is possible, so that the machining speed of veneer is to be set at a value lower than practically required, or the incremental ratio of the required revolution is limited so that the minimum axial diameter of a log must be greater than the one usually used. The actuality, therefore, is such that the processing must be made at a constant speed which is not necessarily practical.
Furthermore, in some veneer lathes of the peripheral driving type, a structure is employed which is designed to interpose an overrunning mechanism in the driving system of the spindle, drive it rotatably at a random lower revolution that corresponds to the log-driving speed of the peripheral driving mechanism, and exert a follow-up rotation of the spindle via the log only at the time of operation of the peripheral driving mechanism. However, if a structure of this kind is provided with the conventional double spindle, the load of an inertial resistance of the large spindle still remains great when the small spindle is used, and the resulting power supply to the outer circumferential surface of the log tends to be extremely greater. Consequently, the damages on the surface of the log become vitaly conspicuous.
On the other hand, the known veneer lathe is so structured usually that a pair of left and right spindles are synchronously connected for rotation through a connecting rod and that both of the double spindles are relatively arranged with a step feeding mechanism which follows the rotation of both the double spindles when it is operated. If the axial portion of a log is broken by an excessive supply of driving power given thereto in the cutting process, the step feeding mechanism still follows the rotation of both of the double spindles irrespective of the difference which occurs between the revolutions of both of the double spindles and the log. As a result, there occurs an abnormal pressure between both of the double spindles and the step feeding mechanism, resulting in an undue wearing of such members as bearings, lead screw, and others and the breakage of an entire log.